In optical measurement technical fields or optical communication technical fields, optical waveguide elements having optical waveguides formed in a substrate having an electro-optic effect such as optical modulator or optical switches are frequently used. Generally, these optical waveguide elements are housed in a package (case) to be sealed and configure an optical-waveguide-element module.
As described in Patent Literature No. 1, a relay substrate (a connecting substrate) for electrically connecting input lines from the outside to a control electrode in an optical waveguide element or a terminal substrate (a connecting substrate) which is electrically connected to the output side of the control electrode in the optical waveguide element and is intended to absorb or lead out output signals to the outside of a module is housed in the case of the optical-waveguide-element module.
Generally, in order to decrease discontinuity in a connecting portion and improve frequency characteristics, as illustrated in FIG. 1, a design is made to be the same electrode dimensions or the same spaces between ground electrodes (GND), between a substrate for modulation and the relay substrate (CP1) which constitute an optical waveguide element 1. It can be also applied to be the same as the above on connecting portion between the terminal substrate (CP2) and the substrate for modulation. When the dimensions or spaces of electrodes are matched to each other on the substrate for modulation and the connecting substrate as described above, it is necessary to prepare as many connecting substrates as the kinds of optical waveguide elements, which causes an increase in manufacturing costs.
In FIG. 1, as the optical waveguide element 1, optical waveguide, not illustrated, is formed on a substrate, and a control electrode constituted of a signal electrode SL and a ground electrode GD is provided in order to control light waves that propagate through the optical waveguides. Optical fibers (FB1 and FB2) are connected to the optical waveguide element 1, input light is introduced into the optical waveguide element 1, and furthermore, output light is led out.
A modulation signal IS is input using a connector CN1, and a modulation signal OS is led out using a connector CN2. A signal line SL1 and a ground line GD1 are formed in the relay substrate (connecting substrate) CP1, and a signal line SL2 and a ground line GD2 are also provided in the terminal substrate (connecting substrate) CP2.
The connector and the signal line on the connecting substrate, or, the connector and the ground line on the connecting substrate are electrically connected to each other directly or using wires RB such as gold ribbon or gold wire. Between the signal line on the connecting substrate and the signal electrode in the optical waveguide element or between the ground line on the connecting substrate and the ground electrode in the optical waveguide element are electrically connected to each other using wire RB such as gold wire.
The optical waveguide element 1 or the connecting substrates (CP1 and CP2) are housed in a metal case 2. The optical fibers (FB1 and FB2) or the connectors (CN1 and CN2) are disposed so as to penetrate through the case.
As the transmission rate (bandwidth) of optical waveguide element is progressed to be higher and wider, a material with lower permittivity than the substrate constituting optical waveguide elements (substrate for modulation) such as alumina has been used for the connecting substrate shown in FIG. 2. In addition, in order to prevent frequency characteristics from being deteriorated due to substrate mode or the like, the dimensions of connection portion in connecting substrates or the like become decreased. Therefore, when the space between ground electrodes or the space between ground lines are configured to be equal each other as in the related art, the signal electrode width in optical waveguide element becomes small, and the width of signal line in the connecting substrate becomes different from one in the substrate having an electro-optic effect such as LiNbO3 or semiconductor substrate, and thus discontinuity of connecting portion is caused, and electrical characteristics are deteriorated. In addition, when the width of signal line or the space between ground lines abruptly change between the input side and the output side on the connecting substrate, impedance mismatch is likely to occur, and deterioration of electrical characteristics become more remarkable.
Furthermore, as illustrated in FIG. 2, in a case in which, in order to match the impedances among signal line outside the case, the connecting substrate and the optical waveguide element, the width S1 of the signal electrode SL in the optical waveguide element and the width S2 of the signal line SL1 (SL2) on the connecting substrate make different each other, and the space W1 between the ground electrodes GD in the optical waveguide element and the space W2 between the ground lines GD1 (GD2) on the connecting substrates make different each other, discontinuity of electric connection, for example, caused by differences of electric field intensities between signal portion and grounding portions on the connecting substrate, and, the optical waveguide element is caused, and deterioration of electric characteristics is occured.
In order to solve the above-described inconvenience, Patent Literature No. 2 makes effort about the disposition of wires electrically connecting between ground electrodes in optical waveguide element and ground lines on connecting substrate.
Meanwhile, the control electrode provided in the optical waveguide element has a number of curved sections from the relation with the arrangement of the signal electrodes. As illustrated in FIG. 2, the length from a point X1 (Y1) which is terminal ends of the ground line GD1 and the signal line SL1 on the connecting substrate to a starting point X2 (Y2) of an operating part in which the control electrode in the optical waveguide element applies an electric field to the optical waveguides along the signal electrode SL becomes a dotted line X(Y). As shown in FIG. 2, the lengths of the dotted line X and the dotted line Y are different each other. Particularly, since the distance from the point X1 to an point along the dotted line X in the curved section and the distance from a point Y1 to an point along the dotted line Y in the curved section are different each other in the respective ground electrodes which put the signal electrode therebetween, the propagation times of microwaves are different, and a local potential difference is occurred between the ground electrodes. Therefore, the propagation characteristics of electrical signals such as microwaves and the like are deteriorated.
Furthermore, in a case in which the space W1 between the ground electrodes which put the signal electrode in the optical waveguide element therebetween and the space W2 between the ground lines which put the signal line therebetween on the connecting substrate are different each other as in a case in which a common connecting substrate is used for different optical waveguide elements, not only including the above-described cause due to the curved section of the signal electrode, but the lengths of the ground electrodes along the signal electrode reaching to the operating part is also more likely to become different. Therefore, deterioration of the propagation characteristics of electrical signals becomes more significant.